1. Technical Field
The present invention generally relates to photolithographic projection of a pattern onto a substrate coated with a light-sensitive material. More particularly, the present invention relates to a mask pattern resulting in an imaged feature that can be measured to monitor exposure.
2. Background Art
As photolithography has progressed, feature size or linewidth has been reduced to a point where precise measurement instrumentation, such as a scanning electron microscope, is needed to measure same. However, such measurement instrumentation is expensive and slows down the production process, for example, in semiconductor fabrication. In addition, when the measurement indicates a feature size outside the acceptable range from the design, one cannot be completely sure whether it is due to exposure or focus, since both affect the size of the printed image. Thus, it would be helpful if there was a way to isolate the effects of exposure from focus on feature size.
In the past, partially transmitting regions of a lithographic mask have been used to achieve a range of exposures from a single mask for resist sensitivity characterization studies. For example, masks have been made with large, spaced partially transmitting windows, several millimeters in size, to provide a discrete range of exposure levels. The exposure was then calibrated according to when a particular patch was developed relative to the others. However, the size and spacing of the partially transmitting windows preclude their use on production wafers and may not provide an optimum exposure setting for the particular feature size desired.
In addition to the partially transmitting windows, at least one attempt has been made to achieve a gray scale mask structure for monitoring exposure. A mask was created with a range of line sizes from the center of the structure to the edge on either side thereof, keeping a constant pitch. When the structure is imaged, the small lines on the mask, smaller than the ability of the lithographic optics to resolve, cause the lines to appear to blur together to create a gray scale feature in the resist on the semiconductor wafer. This process can be analogized to newspaper recreations of photographs, where black and white dots are spaced such that the resolution of the human eye from a reading distance interprets them as shades of gray. However, the effectiveness of this method is reduced as the resolution of the lithographic tool is improved. In addition, the small lines strain the ability of the mask making equipment, and the equipment that verifies the mask pattern may not be able to discern such small lines.
Thus, a need exists for a way to monitor exposure effects on feature size without intervening focus effects and without depending on slow and expensive measurement tools.